(a) Field of the Invention
This invention relates to an improvement of the conditioning step of the hot water process for extracting hydrocarbons or bitumen from bituminous sands (commonly and hereafter referred to as `tar sands`). More particularly, it relates to a technique for controlling the addition of process aid to the conditioning operation with a view to minimizing aid consumption while maximizing bitumen recovery.
(b) Description of the Prior Art
Upgraded hydrocarbons are commercially recovered from tar sands by a three part process involving: (1) mining the whole tar sand; (2) extracting the bitumen from the whole tar sand using the hot water process; and (3) upgrading the recovered bitumen to useful product form. This overall operation is amply described in the patent and other technological literature.
In the first step of the hot water process, the as mined tar sand is `conditioned`. This involves mixing it with hot water, limited quantities of steam to provide temperature control, and one or more chemical `process aids` such as sodium hydroxide or sodium silicate. The conditioning operation is carried out in a large rotating horizontal cylindrical tumbler. The retention time of the mixture in the tumbler is something less than 10 minutes. The conditioning step effects a preliminary separation of the three main components of tar sand (i.e. solids, water and bitumen) one from the other.
The product of the conditioning step is a porridge-like slurry. This slurry is screened, to remove oversize rejects, and then diluted with additional hot water. The screened diluted slurry is introduced into a thickener-like vessel commonly called a primary separation vessel (PSV). Here, the bulk of the bitumen floats to the surface of the vessel contents and forms primary bitumen froth, which is recovered. A solids-rich primary tailings stream is withdrawn from the base of the primary separation vessel and pumped to waste disposal. To gain a further yield of bitumen, a stream of middlings is withdrawn from the midsection of the primary separation vessel. These middlings are treated in subaerated flotation cells to produce a secondary bitumen froth and a secondary tailings stream (which also goes to waste disposal). The froths are subsequently combined, contained water and solids are removed, and the clean bitumen is subjected to upgrading.
As previously stated, the present invention is concerned with the rate of addition of the process aid(s) to the conditioning step. The purpose of the process aid(s) is to assist in separating the bitumen from the mineral matter in the conditioning step. It has been commonly stated in the prior art that the quantity of process aid is determined by controlling the pH of the middlings in the primary separation vessel to a narrow range. For example, Canadian Pat. No. 841,581 to Floyd et al recommends adding process aid so as to obtain a middlings pH range of 7.5 to 9.0, preferably 8.0 to 8.5.
A method of anticipating pH changes due to the buffering action of clays present in tar sand has been described in Canadian Pat. No. 922,254, issued to White and Loveland. These workers measured the aluminum content of the tar sand and related that to the clay content and thereafter practised maintaining a constant middlings pH by regulating water and sodium hydroxide addition according to clay content. This method again results in limiting primary separation vessel middlings pH to a narrow range. Working along similar lines, Graybill and White (Canadian Pat. No. 889,823) measure middlings density to control water addition. Addition of a constant dosage of process aid based on tar sand feed has also been used in an attempt to keep middlings pH constant.